1. Field of the Invention
The present invention relates to a semiconductor device including an electrostatic discharge (ESD) protective element formed between an external connection terminal and an internal circuit region so as to protect an internal element formed in the internal circuit region from an ESD breakdown.
2. Description of the Related Art
In a semiconductor device including a metal oxide semiconductor (MOS) transistor, a so-called off transistor, whose gate potential is fixed to a ground (Vss) to hold an off-state in case of an n-type MOS (NMOS) transistor, is used as an ESD protective element for preventing a breakdown of an internal circuit due to static electricity invading from a PAD terminal for external connection. In order to prevent an ESD breakdown of an internal element, it is important to draw as much part of an electrostatic pulse as possible into the off transistor to inhibit the electrostatic pulse from propagating to the internal element, or to change a fast and large electrostatic pulse into a slow and small signal before transmitted to the internal element.
However, the off transistor needs to flow a large amount of current caused by the drawn static electricity at one time unlike the MOS transistors forming the internal circuit such as a logic circuit, thus the transistor width W is designed to have as large as several hundred micrometers in many cases. As a result, a large occupation area is needed for the off transistor, preventing the total area reduction especially in a small integrated circuit (IC) chip, also becoming a factor for increase in cost of the entire IC.
Further, in many cases, the off transistor often takes a form in which a plurality of drain regions, source regions, and gate electrodes are combined in a comb shape. This structure involving the combination of a plurality of transistors has a difficulty in uniform operation in the entire NMOS transistor for ESD protection. For example, current concentration caused in a portion closer to the external connection terminal may result in the breakdown of the off transistor without sufficient exertion of the original ESD protection function.
As a countermeasure against the above-mentioned problems, there is proposed an invention in which a distance between a contact hole formed on a drain region and a gate electrode is made smaller as a distance from the external connection terminal becomes longer to accelerate the operation of the transistor (for example, refer to FIG. 2 of JP 7-45829 A).
However, when the width W is made smaller for the purpose of a smaller occupation area of the off transistor, the protection function can not be sufficiently accomplished. Further, in the method of JP 7-45829 A, the distance between the contact hole in the drain region and the gate electrode is adjusted, to thereby locally adjust a transistor operation speed. However, the method has problems that a desired contact width cannot be ensured along with a reduction in width of the drain region, and that wiring resistance has been made low through wiring structure including a refractory metal in recent years to thereby accelerate a propagation speed of surge, causing a case in which the transistor operation speed cannot be adjusted only by the distance between the contact hole and the gate electrode. Further, in JP 7-45829 A, there is not disclosed a method for drawing as much part of the electrostatic pulse as possible into the off transistor without propagating the electrostatic pulse to the internal element, or a remedial measure for changing a fast and large electrostatic pulse into a slow and small signal before transmission, in order to prevent the ESD breakdown of the internal element.